1. Field of the Invention
This invention relates to the composition of fluorophosphate optical glass holding high anomalous partial dispersion and having a relatively high refractive index, and the optical constants thereof are such that the refractive index (.eta.d) amounts to 1.58-1.70 and the Abbe number (.nu.d) amounts to 39-70.
2. Description of the Prior Art
Optical glass having anomalous partial dispersion is very important to optical systems and has high utility since it makes modification of secondary spectrum possible in lens designing. Numerous reports have already been made of fluorophosphate glass having anomalous partial dispersion and having a refractive index (.eta.d) less than 1.55 and, in practice, such glass has recently been used for telescope objective lenses or photographic lenses or the like. On the other hand, among lens designers, there has heretofore been a strong demand for glass having a higher refractive index (.eta.d&gt;1.58) and dispersion (.nu.d&lt;70) and also holding anomalous partial dispersion. As optical glass having such optical properties, a glass composition comprising MgF.sub.2 0.1-1.5, CaF.sub.2 0.1-1.5, SrF.sub.2 0-12.5, AlF.sub.3 0-10.0, LaF.sub.3 0.1-1.5, Al(PO.sub.3).sub.3 8.0-12.0, Mg(PO.sub.3).sub.2 3.0-25.0, Ba(PO.sub.3).sub.2 10.0-18.0, BaO 26.2-55.0, B.sub.2 O.sub.3 0-9.0, GeO.sub.2 0-10.0, TiO.sub.2 0-4.0 and Nb.sub.2 O.sub.5 0-20.0 (all being percent by weight) is known, for example, from Japanese Patent Publication No. 28169/1978 (corresponding to U.S. Pat. No. 3,954,484). However, this glass is relatively high in liquid phase temperature and unstable for devitrification, and is not suitable for production on an industrial scale. Further, the great BaO content in this glass leads to high dispersion and insufficient anomalous partial dispersion.